Near the end of the film after McCloud has killed the Kurgan (or however you spell it)in the warehouse, he gets the prize and is lifted into the air supposedly by the forse of the prize - the wires lifting him are clearly visible.
It's mentioned in the commentary. The animation was designed specifically to hide the line holding him up, but it still shows pretty clearly. Of course, as they knew about it, and there could easily be an explanation (How do we know that there isn't a glowing energy line in a final Quickening?), it could be argued that this is no mistake.